Epitaxy, epitaxial growth, and epitaxial deposition refer to growth or deposition of a crystalline layer on a crystalline substrate. The crystalline layer is referred to as an epitaxial layer. The crystalline substrate acts as a template and determines the orientation and lattice spacing of the crystalline layer. The crystalline layer can be, in some examples, lattice matched or lattice coincident. A lattice matched crystalline layer can have the same or a very similar lattice spacing as the top surface of the crystalline substrate. A lattice coincident crystalline layer can have a lattice spacing that is an integer multiple of the lattice spacing of the crystalline substrate. The quality of the epitaxy is based in part on the degree of crystallinity of the crystalline layer. Practically, a high quality epitaxial layer will be a single crystal with minimal defects and few or no grain boundaries. Traditionally, metal contact layers are applied to an epitaxial structure at some point in the upstream processing. With today's complex epitaxial structures often incorporating more than one device functionality, this can require extensive etching and deposition of metals on wafers with a large amount of topography.
Interactions between metals and semiconductors are often critical to device operation. Generally, metal-semiconductor junction barrier height is defined by the metal work function and semiconductor electron affinity. For example, for n-type semiconductor, q×φn=q(φm−χ). And for p-type semiconductor, q×φp=Eg−q(φm−χ), where, q denotes the elementary charge (electron charge); φBn and φBp denote potential barrier height for metal junction with n-type and p-type semiconductor, respectively; φm denotes the metal work function; and χ denotes the electron affinity for semiconductor. Traditionally, a device that is formed with a single metal element is limited to the work function of the particular metal, and thus has a fixed metal-semiconductor junction barrier height, which limits the applicability of the formed device.